Why extending controller in magento is different from model

Question

I want to know why are the steps to extend magento controller different from model or block. Why cant a controller be extended like other magento classes are overridden? Why do we have to include the class file in the file which is extending it in case of controller?

Answer 1

In short: This is how Magento was designed (not to allow override the controller as with Model, Helper and Block). I mean they are free to design the whatever they want.

In long: You can always instantiate Model, Block, Helper via Factory method, usually, Mage::getModel('your/model'), Mage::getBlock('your/block') and Mage::getHelper('module/helper'). However, it is not possible to instantiate controller this way.

The factory method Mage::getModel() checks all configurations files (usually files in et/config.xml of all modules) to determine overloading. However, Controller class are determined by Router based on complex, multi-level system routing.